Difficult technological challenges facing microprocessor design are forcing a rethinking of how best to utilize the increasing number of transistors provided by Moore's Law. One promising approach is to tailor system characteristics to specific applications. These new "hybrid" system designs combine multiple types of processor cores on a single chip to reduce system power and chip area while improving performance. The twelve papers in this issue describe hybrid system challenges and solutions from component design and optimization to system architectures, design tools, programming models, and applications. This issue explores hardware and software design for hybrid systems, ranging from the first supercomputer to achieve a performance of over one petaflops to medical image processing applications.